Such pumping installations usually comprise a piston pump which is lowered inside the well to a specified depth. Its cylinder is securely fixed to the casing of the well while its piston is free to move and is connected to the actuator unit situated on the surface (ground or off-shore platform) by a string of pumping rods constituted by sections that are easily connected end to end or disconnected. The piston generally has non-return valves, and its stroke and frequency are a function of the flow rate of the well.
In all prior wells so far fitted with pumping units, the prime mover is rotary (thermal or electrical). Motion is then transformed either mechanically (nodding beams) or else hydraulically (pump units having actuators). The drive system connecting the prime mover to the string of rods includes members that need to be maintained and adjusted, thus requiring on-site maintenance during extraction, and under current operating conditions the cost of such maintenance is becoming an ever greater burden, particularly in the oil industry.